Recording media (optical disks) such as DVDs (Digital Versatile Discs) and BDs (Blu-ray® Discs) are extensively used as optical disks (media) for recording diverse content including movies, pieces of music and the like.
Today's recording media such as BDs mostly record HD images, i.e., so-called Hi-vision-ready 2K images. As enhancement of image quality continues, those media having ultra-high definition images (UHD: Ultra High Definition images) recorded thereon are expected to multiply. It is to be noted that, for example, 4K and 8K high-definition images are typical of the ultra-high definition images (UHD images).
Because such high-definition image data contains huge amounts of information, the disks such as SDs are required to record information thereon with higher density than ever.
Performed on the optical disks such as BDs is the process of emitting a laser beam to the data-recording surface of the disk and analyzing reflected light therefrom to read the signal recorded on the disk.
However, this method of reproduction using an optical pickup is subject to resolution-level constraints stemming from diffraction of light, for example.
If it is assumed that λ stands for the wavelength of the laser beam and NA for the numerical aperture of the lenses constituting the optical pickup, then resolution limit, i.e., maximum reproducible period, is defined as λ/2 NA.
That is, if the interval between marks (pits) on the optical disk is set to be less than or equal to λ/2 NA, which is the resolution limit, then reproduction using the optical pickup is not feasible. It is to be noted that, for example, PTL 1 (JP 1991-93058A) describes existing techniques disclosing a high-density recording structure of disks.